1. Field of the Invention
The present invention relates to an electric dust collector.
2. Description of the Related Art
An example of a conventional electric dust collector is schematically illustrated in FIG. 7.
In the electric dust collector, as shown in FIG. 7B, a pulse voltage produced by a pulse power supply circuit 142 is superposed on a DC base voltage produced by a base power supply circuit 141 through a coupling capacitor 133 and the superposed voltage is applied to a discharge electrode 130B disposed in a dust collection chamber 130A.
A waveform of the applied voltage is shown in FIG. 7B and has a pulse width of 50 to 200 .mu.s and a pulse frequency of 25 to 400 pps.
Further, in FIG. 7A, numeral 131 denotes a pulse forming capacitor, 132 a switching circuit, 134 a coupling transformer, 135 and 136 an electric power adjustment circuit, 137 and 138 a transformer, and 139 and 140 a rectifier circuit.
FIG. 8A schematically illustrates another conventional electric dust collector.
In the electric dust collector, as shown in FIG. 8A, a DC high voltage produced by a DC high voltage power supply 146 is superposed on a pulse voltage from a pulse forming capacitor 147 without intervention of a coupling capacitor and a discharge electrode 144B disposed in a dust collection chamber 144A is charged with a voltage having a waveform shown in FIG. 8B.
Further, in FIG. 8A, numeral 143 denotes a base voltage supply resistor, 145 a controller, and 148 a switching circuit.
FIG. 9A schematically illustrates still another conventional electric dust collector.
As shown in FIG. 9A, the electric dust collector includes a base power supply circuit 150 and a pulse power supply circuit 152, and a pulse forming capacitor 151 also has the function of a coupling capacitor.
A discharge electrode 153B disposed in a dust collector 153A is charged with a voltage having a waveform shown in FIG. 9B.
Further, in FIG. 9A, numeral 154 denotes a switching circuit.
FIG. 10A schematically illustrates still another conventional electric dust collector.
As shown in FIG. 10A, the electric dust collector includes a pulse generation circuit 164 having a pulse forming capacitor 161 and a high voltage switching circuit 162, and the pulse forming circuit 161 is charged by a DC high voltage power supply 160.
When the voltage at the pulse forming circuit 161 reaches a sufficiently a high value, the switching circuit 162 carries out switching to generate an LC resonance, so that a steep pulse voltage shown in FIG. 10B is superposed on a remaining voltage of a discharge electrode 163B disposed in a dust collection chamber 163A.
Problems of the conventional electric dust collectors are as follows:
In the electric dust collector shown in FIG. 7A;
(1) Since a three-phase AC power is used in the base power supply circuit 141 so as to make the base voltage as smooth as possible, the configuration is complicated, large and expensive. PA1 (2) Further, in order to increase the charging efficiency of the pulse forming capacitor 131, the three-phase AC power is used in the pulse power supply circuit 142, but this results in only a limited improvement in the efficiency. PA1 (3) Since the direction of a base current flowing in a secondary winding of the coupling transformer is the same as that of a pulse current flowing in a primary winding of the coupling transformer 134, the directions of magnetic fluxes generated by the currents are the same. Accordingly, it is necessary to avoid the saturation of the coupling transformer 134 by making the iron core of the coupling transformer 134 extremely large, and a steep pulse having a short pulse width cannot be obtained. PA1 (1) Since the single DC high voltage power supply 146 is used and the DC high voltage power supply 146, the switching circuit 148 and the discharge electrode 144B are always connected electrically, the base voltage is apt to be influenced by electric charges of the discharge electrode 144B and is not smoothed. PA1 (2) Since the applied voltage of the pulse forming capacitor 147 is substantially equal to the applied voltage of the dust collection chamber 144A, that is, the base voltage, the peak value of the pulse voltage is influenced by the base voltage, and therefore the base voltage and the pulse voltage cannot be controlled independently. Accordingly, when the pulse voltage is superposed on the base voltage, there is a possibility that abnormal discharge occurs. PA1 (3) Since the energy of the base current generated by the DC high voltage power supply 146 and the resonance current generated by the discharge electrode 144B is consumed by the base voltage supply resistor 143, the resistor 143 has to have a large capacity. In addition, energy losses increase and this is certainly not desirable in view of the saving of energy. PA1 (1) The base power supply circuit 150 and the pulse power supply circuit 152 are provided independently, while since an end of the pulse forming capacitor 151 is connected to the discharge electrode 153B, the applied voltage of the pulse forming capacitor 151 is influenced by ripples of the base voltage and accordingly the pulse voltage cannot be controlled independently. PA1 (2) In order to avoid the influence of ripples in the base voltage, it is necessary to increase the base voltage, while when the base voltage is increased, abnormal discharge occurs which is undesirable in view of the dust collection performance. PA1 (3) Since a sum of the base voltage and the pulse voltage is applied to the pulse forming capacitor 151, it is necessary to increase its capacitance in order to increase the maximum allowable voltage of the pulse forming capacitor 151 and increase the peak voltage of the pulse upon switching. PA1 (1) Since the switching function and the insulation function are provided in the pulse generation circuit 164, the pulse voltage can be controlled independently. However, since the base power supply is not provided, the base voltage cannot be controlled during the non-pulse period. PA1 (2) Since the resonance current flows during the period when the switching circuit 162 is on, the pulse voltage being attenuated is applied to the discharge electrode 163B plural times, and accordingly the pulse frequency cannot be controlled exactly. PA1 (3) The energy of the resonance current is mainly consumed in the dust collection chamber 163A. However, since a plurality of pulses are applied, more energy is consumed as compared with a single pulse. This is not desirable in view of the dust collection performance.
In the electric dust collector shown in FIG. 8A;
In the electric dust collector shown in FIG. 9A;
In the electric dust collector shown in FIG. 10A;